In recent years, awareness of the need to reduce global warming has increased. To reduce carbon dioxide emissions from vehicles, fuel cell powered vehicles and hydrogen vehicles using hydrogen as fuel such as a hydrogen engine automobile are being developed. A hydrogen storage tank for storing hydrogen gas is installed in hydrogen vehicles. There is a known hydrogen storage tank that accommodates, for example, powdered hydrogen absorption alloy in a tank body.
Patent document 1 discloses a hydrogen storage tank including a hydrogen supply passage and a hydrogen release passage extending from a tank body. Hydrogen gas is supplied to the tank body from a hydrogen supply unit installed at a hydrogen station through the supply passage, and the hydrogen gas in the tank body is discharged through the discharge passage and supplied to a fuel cell. An open-close valve for opening and closing a passage is arranged in each passage. The supply of hydrogen gas to the tank body and the discharge of hydrogen gas from the tank body are carried out by opening and closing each passage with the corresponding open-close valve.
In the hydrogen storage tank accommodating the hydrogen absorption alloy, a filter is attached to the outlet of the supply passage and to the inlet of the discharge passage so that the hydrogen absorption alloy does not move out of the tank body and into the passages. However, a certain type of hydrogen absorption alloy is pulverized to a size of about 1 μm and thus cannot be completely kept in the tank with only the filters. The hydrogen absorption alloy that enters the passages deposits in the open-close valve. As a result, the open-close valve may fail to function properly. Further, an electromagnetic valve is often used as the open-close valve. However, such an electromagnetic valve may have an electrical failure such as a disconnected wire. An open-close valve must be replaced if it fails to function.
The hydrogen absorption alloy has a property of being easily oxidized when exposed to air. Thus, when replacing the open-close valve, a generally employed method would be to completely discharge the hydrogen gas from the tank body and then supplying and filling the tank body with inert gas. However, in such a method, the open-close valve is removed after the inert gas is supplied. Thus, there is a possibility that air may enter the tank body if the inert gas leaks out from the tank body before completing the replacement of the open-close valve. Therefore, there still is a possibility of the hydrogen absorption alloy in the tank body being exposed to air, and this method is thereby inadequate as a means for inhibiting oxidation and deterioration of the hydrogen absorption alloy.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-139298